MySQL中的交易隔離級別是多少？默認值是哪個？

MySQL的默認事務隔離級別是可重複讀（Repeatable Read），它通過MVCC和間隙鎖防止臟讀和不可重複讀，並在大多數情況下避免幻讀；其他主要級別包括讀未提交（Read Uncommitted），允許臟讀但性能最快，1. 讀已提交（Read Committed）確保讀取已提交數據但可能遇到不可重複讀和幻讀，2. 可重複讀（Repeatable Read）默認級別，保證事務內多次讀取結果一致，3. 串行化（Serializable）最高級別，通過鎖阻止其他事務修改數據，確保數據完整性但犧牲性能；可通過SELECT @@tx_isolation; 和SET SESSION TRANSACTION ISOLATION LEVEL來查看或設置隔離級別。

MySQL supports several transaction isolation levels, which determine how transactions interact with each other and how data consistency is maintained. The default isolation level in MySQL is REPEATABLE READ .

Here's a breakdown of the main isolation levels you might encounter:

Read Uncommitted – Lowest Isolation

This level allows one transaction to read uncommitted changes made by another transaction. It can lead to "dirty reads," where data that hasn't been finalized (and might be rolled back) is read.

• Not commonly used in practice due to risk of inconsistency
• Fastest performance-wise, but least safe

Example:
If Transaction A updates a row but hasn't committed yet, Transaction B can see that update immediately—even if it gets rolled back later.

Read Committed – Avoids Dirty Reads

This level ensures that any data read during a transaction is committed at the moment it is read.

• Prevents dirty reads
• May still allow non-repeatable reads and phantom reads

In some databases like Oracle, this is the default, but not in MySQL.

Use case:
When you want to make sure you're only reading committed data, but don't need strict consistency across the entire transaction.

Repeatable Read – Default in MySQL

At this level, MySQL ensures that if a transaction reads the same data multiple times, it will see the same values throughout the transaction.

• Prevents dirty reads and non-repeatable reads
• Also prevents most phantom reads (though technically possible in some edge cases)
• Uses multi-version concurrency control (MVCC) and next-key locks to achieve this

Because it's the default, you'll often find it being used without explicit configuration.

Common behavior:

• If two transactions try to modify the same rows, one will wait or fail depending on locking
• Consistent reads within a transaction use a snapshot of the database

Serializable – Highest Isolation

This is the strictest isolation level. It places a lock on the rows being read, preventing other transactions from modifying them until the current transaction completes.

• Prevents dirty reads, non-repeatable reads, and phantom reads
• Can significantly reduce performance due to increased locking

It's typically used when absolute data integrity is required and performance isn't a top priority.

Summary of Isolation Levels and Behavior

*MySQL uses next-key locking in Repeatable Read to prevent most phantom reads.

You can check or change the isolation level using SQL commands like:

 SELECT @@tx_isolation;
SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;

Just keep in mind that changing the isolation level affects how your transactions behave under load and concurrency.

基本上就這些。

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